I seem to have the same problem, but only when running multiple high useage aps. I have 1.11 FW and A2 HW. When I run P2P (Azureus) with a couple downloads, limited to 30 connection each, with a global max of 50, and when I run a PTPP VPN connection, the VPN drops out intermittently.
I have used the QOS to prioritize (around priority 80) GRE port and the other VPN port, but still no dice. Every 2 to 20 mins there is a VPN drop-out and reconnect. I have since turned it off and found no improvement either way.
I can tell you, this is much better performance than my DI-624, which basically became unusable in the same application. Everything else about the DIR-655 seems a fantastic improvement. Keep up the good work, but any insight on the VPN drop-outs would be great!